Observation Of The Vertical Distributions Of NO₂ And HCHO In The Troposphere At The Longfengshan Background Station

NO₂ and HCHO are important trace gases in the troposphere and have serious impacts on both the ecological environment and human health.The Northeast region is an important industrial and agricultural base in China.However,due to the development of heavy industry,environmental pollution in the Northeast is becoming more and more serious.Therefore,it is important to perform researches on atmospheric pollution in the Northeast background area.The vertical profiles of nitrogen dioxide（NO₂）and formaldehyde（HCHO）in the troposphere at the Longfengshan（LFS）regional atmospheric background station（127°36′E,44°44′N,330.5 m above sea level）from 24 October 2020 to 13 October 2021 were retrieved from solar scattering spectra by multi-axis differential optical absorption spectroscopy（MAX-DOAS）.We analyzed the temporal variations of NO₂ and HCHO as well as the sensitivity of ozone（O₃）production to HCHO and NO₂.By using the QDOAS spectral analysis model,we obtained the spectral analysis parameters and corresponding differential slant column densities（d SCDs）for the Longfengshan regional atmospheric background station:we investigated the effects of pre-calibration spectra,polynomial fitting order,and wavelength window range on spectral calibration;and investigated the differences in the inversion of NO₂ and HCHO d SCDs with different wavelength ranges,molecular absorption cross sections.Then,based on the tropospheric NO₂ and HCHO d SCDs of different elevation angle observation sequences,we use an aerosol extinction and trace gas concentration profile inversion algorithm（Pri AM）based on the optimal estimation method to invert the vertical profiles of NO₂ and HCHO VMRs.By comparing the relative errors of d SCDs and cost functions under different parameter,we determined the best setting parameters for vertical profile inversion at the Longfengshan regional atmospheric background station.The tropospheric NO₂ and HCHO vertical column densities（VCDs）were obtained by integrating the vertical profiles.In this research,we have compared the VCDs of NO₂ and HCHO obtained by the geometric approximation method and profile integration.We found that the levels of VCDs derived by these two methods were largely equivalent.We found that the VCDs and near-surface VMRs for NO₂ were high in the cold months and low in the warm months.The largest NO₂ VMRs occur in the near-surface layer for each month,decreasing with altitude.In April,NO₂ VCDs increased slightly after 14:00 BJ.In July,NO₂ VCDs slightly decreased from 5:00 to 9:00 BJ,stayed almost the same from 10:00 to 15:00 BJ,and then increased slightly after 16:00 BJ.In October,NO₂ VCDs were nearly constant throughout the day.The VCDs and near-surface VMRs of HCHO were lower in colder months and higher in warmer months.In April,July and October,the VMRs of HCHO had elevated layers throughout the day.In April and July,the VCDs of HCHO showed a diurnal variation trend of high in the morning and evening and low at noon,and the variation was not obvious in October.By comparing with the corresponding temperature and relative humidity qualitatively,we found that high NO₂ VCDs tended to occur at low temperatures and high humidity,but this relationship was not found between HCHO and temperature.We used HCHO and NO₂ to represent VOCs and NO_x,respectively,to estimate the sensitivity of ozone production at the Longfengshan regional atmospheric background station.We found that O₃ production is mainly in the NO_x-limited region.The sensitivity of O₃ production to NO₂ and HCHO is closely related to temperature and relative humidity.Air masses from different directions can lead to different sensitivities of O₃ production to NO_x and VOCs at LFS Station.This is the first study to observe NO₂ and HCHO in the northeastern background region by MAX-DOAS,and to investigate the concentration variation and vertical distribution characteristics of NO₂ and HCHO.It is important for enhancing the understanding of background atmospheric chemistry and regional ozone pollution processes.